The present invention relates generally to pressure measuring circuits. More particularly, the invention relates to a temperature compensated pressure measuring circuit having a microprocessor which determines pressure based on oscillating, digital signals input to the microprocessor representative of sensed pressure and temperature. The microprocessor determines pressure in accordance with the input pressure and temperature and outputs a temperature compensated, calibrated pressure signal.
Piezoresistive pressure transducers are commonly used to measure pressure. In a typical application, a Wheatstone bridge is disposed on a thin silicon diaphragm of a piezoresistive sensing die which deflects in response to an applied pressure. When the thin silicon diaphragm of the sensing die deflects, the bridge resistance changes in accordance with the amount of deflection. A current or voltage signal energizes the bridge, and variations in the output signal indicate that the resistance, i.e., pressure on the diaphragm, has changed. The output signal is usually calibrated using calibration values determined on a piece by piece or lot by lot basis during the manufacturing process. In some piezoresistive pressure sensors, the resistance values of the bridge resistors may vary not only with changes in pressure, but also with changes in temperature as well, requiring temperature compensation in the output signal processing circuit.
In a typical analog circuit, temperature compensation comprises placing resistors unaffected by temperature variations in the Wheatstone bridge to significantly reduce temperature effects. Calibration of the piezoresistive sensor is then controlled by adjusting current to the bridge and measuring the bridge values at two temperatures and two pressures. This provides a temperature compensated, calibration curve for generating an output signal. Such a technique works well if the output values of the bridge resistances are fairly linear. However, nonlinearity of piezoresistive bridge resistors exists, especially at cold temperatures. Moreover, many other factors affect the linearity of the piezoresistive die including die size, diaphragm thickness, resistor geometry, resistor location on the diaphragm, and the resistor implant process effect.
The present invention provides a more accurate and efficient approach for performing temperature compensation on pressure measurements affected by temperature variations using a digital pressure sensing circuit. The digital pressure sensing circuit includes a first oscillator circuit for providing an oscillating pressure signal having a frequency varying in accordance with the sensed pressure and a second oscillator circuit for providing an oscillating temperature signal having a frequency varying an accordance with a sensed temperature. A temperature compensation means receives said pressure and temperature signal and provides an output signal in which the temperature based effect of the pressure signal are significantly diminished. A microprocessor receives the temperature compensated signal and generates an output signal varying in accordance with said temperature compensated signal. The output signal is based on stored, predetermined calibration values.
From the subsequent detailed description taken in conjunction with the accompanying drawings and subjoined claims, other objects and advantages of the present invention will become apparent to those skilled in the art.